The aim of this project is to develop and apply methods for identifying transcribed sequences from large chromosomal regions. These methods can be used both in the isolation of disease genes and in making integrated genetic, physical and transcriptional maps. Our approach is to screen large arrayed genomic libraries with cDNA probes. Basing the transcriptional map on such reference genomic libraries has a number of advantages: physical mapping based on the same clones obviates the need for mapping each expressed sequence clone separately, and, in contrast to cDNA library approaches, only one reference library is needed for identifying transcribe sequences in any number of tissues and developmental stages. We have concentrated our efforts on human chromosome 21. First, our data on chromosome 21 can be integrated with our previous results on the syntenic mouse chromosome 16. Second, trisomy of chromosome 21 results in pleiotropi developmental and functional defects of Down syndrome. We have used two arrayed genomic libraries from chromosome 21. First, as part of the Joint YAC Screening Effort on Chromosome 21 we have isolated yeast artificial chromosome (YAC) clones for the region of chromosome 21 bands 21q22.2-3 between D21S55 and D21S3. These YAC clones cover 1.4 Mb of the approx. 2 Mb between these markers. The human DNA inserts from these YAC clones were subcloned into a lambda vector and arrayed. The second library is the chromosome 21 cosmid reference library from Hans Lehrach (ICRF, London). Screening the reference library with cDNA probes from cerebral cortex and from a T-cell line has identified 886 expressed sequences. We are now identifying exon sequences by cDNA screening of subcloned fragments and by exon trapping protocols.